The Cell Cycle

Chapter 12

• Cell division - process cells reproduce; necessary to living things.

• Cell division due to cell cycle (life of cell from origin in division of parent cell until own division into 2)

• Unicellular organisms - results in many new members.

• Multicellular organisms - division helps in development of organism and repair and renew preexisting cells

• Requires distribution of identical genetic material (DNA) to 2 daughter cells.

• Genome - cell’s genetic information packaged as DNA.

• DNA molecules packaged into chromosomes.

• Body cells - somatic cells; sex cells - gametes.

• DNA has proteins – maintains structure; helps control gene activity.

• Duplicated chromosome - 2 sister chromatids (identical copies of chromosome’s DNA)

• Region where strands connect shrinks to narrow area (centromere)

• Processes continue every day to replace dead and damaged cells.

• Produce clones - cells with same genetic information.

http://www.s8int.com/images2/cloned.jpg

Cloned cells

• Mitotic (M) phase of cell cycle alternates with much longer interphase.

• M phase includes mitosis, cytokinesis.

• Interphase - 90% of cell cycle.

• Interphase - cell grows by producing proteins and cytoplasmic organelles, copies chromosomes, prepares for cell division; 3 subphases.

• 1G1 phase (“first gap”) - growth. • 2S phase (“synthesis”) -

chromosomes copied. • 3G2 phase (“second gap”) - cell

completes preparations for cell division.

http://www.fhcrc.org/science/labs/fero/RL_gifs/cycle.jpg

• Mitosis – 5 subphases.• End interphase - centrosomes

duplicated, begin to organize microtubules into aster (“star”).

• 1Prophase - chromosomes tightly coiled, with sister chromatids joined together.

• Nucleoli disappear; mitotic spindle forms, appears to push centrosomes away toward opposite ends (poles) of cell.

• 2Prometaphase - nuclear envelope fragments and microtubules from spindle interact with chromosomes.

• Microtubules from 1 pole attach to 1 of 2 kinetochores (special regions of centromere), microtubules from other pole attach to other kinetochore.

• 3Metaphase - spindle fibers push sister chromatids until all arranged at metaphase plate (imaginary plane equidistant between poles)

• 4Anaphase - centromeres divide, separating sister chromatids.

• Each pulled toward pole to which it is attached by spindle fibers.

• 2 poles have equivalent collections of chromosomes.

• 5Telophase - cell elongates; free spindle fibers from each centrosome push off each other.

• 2 nuclei form, surrounded by fragments of parent’s nuclear envelope.

• Cytokinesis (division of cytoplasm) begins.

• Animals - cytokinesis (cleavage) - appearance of cleavage furrow in cell surface near old metaphase plate.

• Cytoplasmic side of cleavage furrow contractile ring of actin microfilaments and motor protein myosin form.

• Contraction of ring pinches cell in 2.

• Plants, cytokinesis - cell plate between dividing cells.

• Plate enlarges until membranes fuse with plasma membrane at perimeter; contents vesicles forming new wall material in between.

Bacteria• Prokaryotes - binary fission.• DNA of bacteria coiled, highly

packed. • Binary fission - chromosome

replication begins at 1 point in circular chromosome, (origin of replication).

• Copied regions move to opposite ends of cell.

• As chromosome replicates and copied regions move to opposite ends of cell, bacterium grows until it reaches 2x original size.

• Cell division involves inward growth of plasma membrane, dividing parent cell into 2 daughter cells with complete genome.

Regulation of cell cycle

• Some cells divide frequently in life (skin cells), others can divide (reserve - liver cells) mature nerve, muscle cells do not divide at all.

• Some control over when cells divide/how often they divide in lifetime.

http://www.ii.bham.ac.uk/webs/shuttleworth/bbsrc1.jpg

• Cycle driven by specific chemical signals in cytoplasm.

• Events of cell cycle directed by cell cycle control system.

• Checkpoint in cell cycle is critical control point where stop/go signals regulate cycle.

• 3 major checkpoints found in G1, G2, and M phases.

• G1 checkpoint (most important), cell either get go ahead to finish cycle and divide, or receive stop signal.

• If stop signal - goes into G0 phase (remains in limbo waiting to start).

• Most human cells in this mode.

http://www.microscopy-uk.org.uk/mag/imgaug99/01.jpg

• Proteins, kinases, can activate/deactivate other proteins.

• Kinases always present in cell; need cyclins (protein) to activate.

• Complex of kinases and cyclin - cyclin-dependent kinases (Cdks).

http://www.mie.utoronto.ca/labs/lcdlab/biopic/fig/9.5.jpg

• MPF (“maturation-promoting factor”) triggers cell’s passage past G2 checkpoint to M phase.

• G1 checkpoint regulated by at least 3 Cdk proteins and several cyclins.

http://www.uic.edu/classes/bios/bios100/summer2002/cdk02.gif

• M phase checkpoint makes sure chromosomes are attached to spindle so each cell ends up with right amount of chromosomes.

• Cell division influenced by growth factors, proteins released by 1 group of cells that stimulate other cells to divide.

http://www.fhcrc.org/science/education/courses/cancer_course/basic/img/growth_factors.gif

• Platelet growth driven by growth factors.

• Presence of injury - released to stimulate division of platelet cells to seal wound.

• Density of cells too high - cell division inhibited.

Cancer

• Cancer cells divide out of control - no regulation.

• Can either produce own growth factors or have problem in signaling pathway.

• Can divide indefinitely if they have continual supply of nutrients.

http://www.sandia.gov/news/resources/releases/2005/images/mitopic.jpg

• Starts when single cell undergoes transformation to change it into cancer cell.

• If immune system does not destroy, can form tumor (gathering of cells).

• If tumor does not invade other areas - benign.

• If it does - malignant.• If cells get into blood stream, travel

throughout body (metastasis).

http://www.livercancer.com/images/metastasis.gif

• http://www.teachersdomain.org/resources/tdc02/sci/life/stru/dnadivide/index.html